The White House on Thursday condemned Russia after the arrest of an American reporter for The Wall Street Journal on espionage charges.
Evan Gershkovich, 31, was detained in the Urals city of Yekaterinburg, Russian authorities said.
His arrest marks the highest-profile detention of an American since Russian authorities arrested basketball player Brittney Griner, who was released in December after spending 10 months in jail on drug charges.
"The targeting of American citizens by the Russian government is unacceptable," White House press secretary Karine Jean-Pierre said in a statement.
"We condemn the detention of Mr Gershkovich in the strongest terms."
Ms Jean-Pierre later called the espionage charges "ridiculous".
Mr Gershkovich has worked in Russia as a journalist for six years.
Russia's FSB security service said it arrested him on suspicion "of spying in the interests of the American government" by collecting information on "one of the enterprises of Russia's military-industrial complex", which it did not identify.
The Wall Street Journal said it “vehemently” denied the allegations and was “deeply concerned for the safety of Mr Gershkovich”.
US Secretary of State Antony Blinken also said he was "deeply concerned" by the arrest.
"In the strongest possible terms, we condemn the Kremlin's continued attempts to intimidate, repress and punish journalists and civil society voices," he said in a statement.
The US has repeatedly warned Americans not to travel to Russia since it invaded Ukraine more than a year ago.
"Americans should heed the US government’s warning to not travel to Russia," Ms Jean-Pierre said.
"US citizens residing or travelling in Russia should depart immediately, as the State Department continues to advise."
International media watchdog Reporters Without Borders (RSF) said it was “alarmed by what looks like retaliation”.
RSF said Mr Gershkovich “was investigating the military company Wagner” — a mercenary group that plays a prominent role in Russia's campaign in Ukraine.
It said he had been asked “by the American side” to gather information on “the activities of one of the enterprises of the military-defence complex”.
“The foreigner was detained in Yekaterinburg while attempting to obtain classified information,” the FSB said. The central Russian city is 1,800 kilometres east of Moscow.
Foreign Ministry spokeswoman Maria Zakharova claimed Mr Gershkovich had been “caught red-handed”.
Kremlin spokesman Dmitry Peskov said: “We are not talking about suspicions — he has been caught in the act.”
Mr Gershkovich reports on Russia and Ukraine as a correspondent in the newspaper's Moscow bureau.
His last report, published this week, focused on the Russian economy's slowdown amid western sanctions imposed when Russian troops entered Ukraine.
The FSB did not say when the arrest took place. He could be sentenced to 20 years in prison if convicted.
The Committee to Protect Journalists (CPJ) called for his quick release.
“By detaining the American journalist Evan Gershkovich, Russia has crossed the Rubicon and sent a clear message to foreign correspondents that they will not be spared from the ongoing purge of the independent media in the country," said Gulnoza Said, CPJ’s Europe and Central Asia programme co-ordinator.
According to the CPJ, at least 19 journalist were in jail in Russia as of December, the last time the press organisation conducted a prison census.
Before joining The Wall Street Journal, Mr Gershkovich worked for AFP in Moscow. He was previously a reporter for The Moscow Times, an English-language news website.
He speaks Russian. His parents live in the US but are from the former Soviet Union.